Cumulonimbus clouds (Cb) are much larger and more vertically developed
than fair weather cumulus.
They can exist as individual towers or form a line
of towers called a squall line.
Fueled by vigorous
convective updrafts
(sometimes in excess 50 knots),
the tops of cumulonimbus clouds can easily
reach 39,000 feet (12,000 meters) or higher.